1. Field of the Invention
This invention relates to a method of driving an electrooptical display device.
Although the electrooptical display devices to which the present invention is directed include all display devices which exhibit a capacitive property, such as a liquid crystal, an EL, and the like, the following description will be of a display device which uses a liquid crystal, by way of example.
2. Description of the Related Arts
FIG. 2 of the accompanying drawings is a wiring diagram showing the relationship between matrix electrodes and a drive circuit of a conventional simple matrix type liquid crystal display device. In this diagram, symbols X1 to Xm denote segment electrode lines; Y1 to Yn common electrode lines; 201 is a segment electrode drive circuit for driving the segment electrode line; 202 is a common electrode drive circuit for driving the common electrode lines; 203 is a control circuit for controlling the segment electrode drive circuit 201 and the common electrode drive circuit 202; and 204 is a drive power supply circuit for generating a power supply voltage for driving the segment electrode drive circuit 201 and the common electrode drive circuit 202 and generating a liquid crystal drive voltage to be applied to the segment electrode line and to the common electrode line through the two drive circuits 201 and 202.
A specific example system of the circuit construction are shown in FIG. 2, and FIG. 3 shows only one thereof, to simplify the explanation. Therefore, although the explanation will be given in detail on the basis of FIG. 3, the technical concept of the present invention also can be effectively applied to the drive circuits of the other systems shown in FIG. 2, for example.
In FIG. 3, a segment electrode drive circuit 301 comprises a logic circuit 305 for processing the signals sent from a control circuit (not shown in the drawing) and an output circuit 302j which selectively supplies +Vs or -Vs to the jth (j=1, . . . , m) segment electrode Xj on the basis of the instruction from the logic circuit 305. Namely, when a transistor 303j is ON and a transistor 304j is OFF, +Vs is applied to the segment electrode line Xj, and when the transistor 303j is OFF and the transistor 304j is ON, -Vs is applied to the segment electrode line Xj. A state wherein the two transistors 303j and 304j are simultaneously ON does not occur. The substrate of these transistors 303j and 304j are connected to positive and negative power supplies +Vd and -Vd, which are applied to the logic circuit 305, respectively (with the proviso that .vertline.Vd.vertline..gtoreq..vertline.Vs.vertline.).
Further, the common electrode drive circuit 306 comprises a logic circuit 311 for processing the signals sent from the control circuit (not shown) and an output circuit 307k which selectively supplies +Vc or -Vc or 0 to the kth (k=1, . . . , n) common electrode line Yk on the basis of the instruction of the logic circuit 311. Namely, when a transistor 308k is ON and a transistor 309k and a (semiconductor) switch 310k are OFF, +Vc is applied to the common electrode line Yk, and when the transistor 308k and the switch 310k are OFF and the transistor 309k is ON, -Vc is applied to the common electrode line Yk. While, when the switch 310k is ON and the transistors 308k and 309k are OFF, zero (0) potential is applied to the common electrode line Yk. A state wherein at least two of the transistors 308k and 309k and the switch 310k are simultaneously ON does not occur. The substrate of these transistors 308k, 309k and the substrate of the transistor that constitutes the switch 310k are connected to the positive and negative power supplies +Ve, -Ve, which are applied to the logic circuit 311, respectively (with the proviso that (.vertline.Vc.vertline..gtoreq..vertline.Vd.vertline.).
Therefore, the difference voltage between the segment electrode drive voltage VXj (+Vs, -Vs) and the common electrode drive voltage VYk (+Vc, 0, -Vc) is applied to the pixel Pjk formed at the point of intersection between the segment electrode Xj and the common electrode Yk, and there are several methods of selecting the timings for selecting each of these voltages.
FIG. 4 is a diagram showing an example of the ideal voltage waveforms to be applied to the liquid crystal by using the circuit construction shown in FIG. 3. In this diagram, periods T1(t1), T2(t2), . . . , Tn(tn) represent those periods in which the first segment electrode Y1, the second common electrode Y2 and the nth common electrode Yn are selectively driven, and the period from the period T1 to the period Tn (or from the period t1 to the period tn) is one vertical scanning period. The respective pixels are driven by the voltage applied during the selection period, which is 1/n of one vertical scanning period, and during the non-selection period which is 1-(1/n) of one vertical scanning period. During the period T1, +Vc is applied to the segment electrode Y1 and the 0 potential is applied to the other common electrodes. During the period T2, -Vc is applied to the segment electrode Y2 and the 0 potential is applied to the other common electrodes. The system which reverses the polarity of the selective drive voltage whenever the row to be driven is selectively changed in this manner is referred to as a "row reversion system". In the subsequent vertical scanning period t1, . . . , tn the polarity of the selective drive voltage to be applied to each row is further reversed, and this system is referred to as a "field reversion system". Accordingly, the system shown in FIG. 4 is referred to as a "row reversion/field reversion system".
Furthermore, the drive voltage applied to the segment electrode is determined in accordance with the data which is to be displayed. Assuming that the pixel portion corresponding to the segment electrode Xa is to be displayed black throughout all the periods, the pixel portion corresponding to the segment electrode Xb is to be displayed white throughout all the periods, and the liquid crystal panel to be used is normally black (which becomes more and more transparent with an increasing applied voltage), then the voltages such as VXa and VXb shown in FIG. 4 are applied to the respective segment electrodes. For example, a voltage VY1-VXa is applied to the pixel Pa1 at the point of intersection between the common electrode Y1 and the segment electrode Xa, and the voltage waveform thereof is represented by VPa1 in FIG. 4. A voltage such as VPb1 shown in FIG. 4 is applied to the pixel Pb1 formed by the common electrode Y1 and the segment electrode Xb.
Assuming that the liquid crystal responds to the effective value of the voltage applied, the effective value of the voltage applied to the pixel Pa1 described above during one scanning period (hereinafter referred to as the "driving effective voltage") is expressed by the formula (1) below and the driving effective voltage applied to the pixel Pb1 described above is likewise given by the formula (2) below: ##EQU1##
The difference between the formulas (1) and (2) given above appears as the difference of the display state (dark and bright). Accordingly, the greater this difference, the better the display, and the condition that provides the best display state is that under which the quotient (Von/Voff) obtained by dividing the formula (2) by the formula (1) becomes the greatest, and is given by the formula (3) below. The quotient (Von/Voff) at this time is given by the formula (4) below: ##EQU2##
The ratio .vertline.Vc.vertline./.vertline.Vs.vertline. is referred to as a "driving voltage ratio" and when the driving voltage ratio satisfies the formula (3), the ratio is referred to as an "optimum driving voltage ratio". The values Voff and Von are determined primarily when Vc and Vs are decided. A driving effective voltage outside this range cannot be applied in principle, but a driving effective voltage between Von and Voff can be applied. An example of the segment electrode driving voltage waveform in this case is represented by VXc in FIG. 4. When such a means is employed, a liquid crystal television apparatus requiring a gradation display can be accomplished.
When the formula (3) is employed, the value .vertline.Vs.vertline. that provides the maximum contrast can be determined when .vertline.Vc.vertline. is set to a certain value, and the contrast drops at values other than this .vertline.Vs.vertline. value. Nevertheless, since the waveforms such as VXa, VXb, VXc, etc., shown in FIG. 4 represent merely the ideal state, and such an ideal state cannot be attained in practice because dull portions (inclusive of spikes) occur in the liquid crystal drive voltage waveform due to the influences of the parasitic resistance existing parasitically at each portion and the capacitance of the liquid crystal. Therefore, even if the drive voltage is set on the basis of the formula (3), to thus obtain the maximum contrast, the contrast that theoretically should be obtained cannot be obtained in practice. Namely, the greater the dullness of the drive voltage waveform applied across both ends of the liquid crystal, the greater the drop in the contrast.
Next, this dullness of the drive waveform leads to a drop in the response of the liquid crystal. Namely, the response of the liquid crystal is increased with a greater Von/Voff value, but if any dullness exists in the waveform, the value Von/Voff becomes smaller and the response of the liquid crystal drops. Accordingly, when a certain display having a quick motion is effected, an "after-image" or "image lag" phenomenon becomes more noticeable. Furthermore, the dullness of the drive waveform results in cross-talk, known conventionally as a critical problem, in the simple matrix type display device. When a display such as that shown in FIG. 5(A) is effected on a liquid crystal television receiver, for example, the practical display image becomes as shown in FIG. 5(B). In a display device of the type wherein a display panel is divided into upper and lower sections, to improve a driving duty ratio, and these upper and lower display panels are driven independently of each other, the display obtained in practice is as shown in FIG. 5(D), when an image as shown in FIG. 5(C) is to be displayed. This is because the dull portions appear in the voltage waveform applied to the liquid crystal, and the ideal state is not attained due to the influences of the output resistance of the drive power supply circuit 204, the internal wiring resistance of the segment and common electrode drive circuits 201, 202, the output resistance thereof, the connection resistance between both drive circuits and the display panel, the resistance of the outgoing electrode portion, and the like, as described above.
Also as described above, the dullness of the voltage waveform applied across both ends of the liquid crystal deteriorates all the characteristics of the liquid crystal display device, and in some cases, exerts an adverse influence such that the liquid crystal display device can no longer be used. Counter-measures employed in the past to solve this problem first stabilize the voltage to be given to the drive circuit from outside and then reduce the resistance of each part as much as possible, but it is practically difficult to make the resistance of each part zero and thus a certain degree of resistance always remains. Accordingly, in many cases the conventional counter-measures do not provide a sufficient effect.
The dullness of the waveform applied across both ends of the liquid crystal deteriorates all the characteristics of the liquid crystal display as described above. In contrast, the present invention is directed to improve the dullness of the waveform by a novel method, and to accomplish an ideal drive state, from all aspects. Since the cross-talk has been primarily discussed as the principal problem resulting from the dullness of the waveform, the explanation will be based mainly on the cross-talk problem, to thus clearly distinguish the present invention from the prior art technique.
A typical conventional explanation of the cross-talk is shown in FIG. 6. Assuming that all the pixels on line A display only white (or black), the column drive voltage of the line A is reversed whenever a row scanning is carried out, and whenever this reversion takes place, a charge/discharge to and from the liquid crystal as the capacitive load is effected. Accordingly, the dullness occurs in the waveform even during the non-selection period of the driving voltage VA applied to both ends of an arbitrary one of the pixels on the line A, as represented by VA in FIG. 6. Also, assuming that the pixels on line B pick up the display state where white and black are reversed at every line, the column drive voltage VB of the line B retains a predetermined value, and therefore, a charge/discharge to and from the liquid crystal during the non-selection period is not effected, and the drive voltage applied across both ends of the arbitrary one of the pixels on the line B becomes VB, as shown in FIG. 6. When the non-selection periods thereof are compared, the effective value of VA is found to be smaller than the effective value of VB, and thus the pixels which should appear at the same brightness are dark in the line A and bright in the line B. The conventional explanation regards this phenomenon as the cause of the cross-talk.
A proposal for an improvement based on the concept described above is disclosed, for example, in Japanese Examined Patent Publication No. 64-4197, and this prior art technique provides certain effects. These prior art inventions, however, are not directed to an improvement of the dullness of the waveform itself, but are directed mainly to making uniform the number of times of a charge/discharge that generates the dullness of the waveform, and further, assume that the display data are binary data (black and white). Accordingly, they are not effective for a gradation display such as a television image.
When the display data is binary data, a switching of the drive voltage conforms with the scanning switching timing of the common electrodes. Therefore, an adjustment can be made so that the effective value of each column at the time of a non-selection becomes uniform, regardless of the display pattern, by applying contrivances to the polarity reversion period of the row drive voltage to substantially equalize the number of times of a charge and discharge of each column at the time of a non-selection. In the liquid crystal television receiver having a gradation, however, a switching of the drive voltage of the segment electrodes does not always coincide with the scanning switching timing of the common electrodes, and thus the number of times of a charge and discharge cannot be adjusted even when the polarity of the row drive voltage is reversed.
The inventor of the present invention carefully examined the influences of the dullness of the waveform on the cross-talk, and found that there are some cases which cannot be fully explained by the concept shown in FIG. 6. The inventor therefore attempted to reproduce the liquid crystal drive state, to thereby analyze such cases. FIG. 7 shows a conventional model as the basis of the explanation of FIG. 6. The basic point in FIG. 7 is that a segment electrode, which originally should exist as a plurality thereof, is represented by one common electrode. Namely, among a plurality of common electrodes, a large voltage is selectively applied to only one electrode during a certain period, and all of the others are fixed at the zero (0) potential. Therefore, the influence of the selected common electrode is excluded by regarding it as sufficiently small as a whole, and an absolute greater number of common electrodes that are in the non-selection state can be collected as one electrode. Then, each segment electrode can be regarded as an aggregate of electrodes each having a capacitance c with respective to one common electrode which is at the zero (0) potential, and these segment electrodes can be regarded as being switched to +Vs and -Vs by the switches S1, S2, . . . each having an output resistance ro.
The problems with this reproduction are that only the resistance component of the segment electrodes is taken into consideration as the resistance component, and further, only the output resistance of the switches (corresponding to the transistors 303j, 304, in FIG. 3) is handled. It is true that the output resistance of the integrated liquid crystal driving circuit is on the order of kilo-ohms, and is by far greater than the resistance of the resistors added, but a resistance (inclusive of the output resistance) also exists in series in the power source line, although its value is small, and the sum of the currents flowing through a plurality of paths are associated with this resistance. Therefore, there may be case where this resistance cannot be neglected.
Particularly, the power supply line resistance involved in driving the segment electrodes is not taken into consideration in the explanation of FIG. 6, but this is believed to be a factor that cannot be neglected when the mode of appearance of the cross-talk in the liquid crystal television image is examined. Therefore, when the resistance of each power supply line is added to FIG. 7, the equivalent circuit becomes as shown in FIG. 8. In FIG. 8, a resistor RD is inserted to the +Vs power supply line, and a resistor RS to the -Vs power supply line. For the common electrodes, a resistor RM is added to the zero potential. This resistor RM includes the output resistance of the common electrode drive circuit (the output resistance of the semiconductor switch 310k in FIG. 3).
Since the cross-talk occurs when the drive waveform of the segment electrodes is different, the case whereby a plurality of segment electrodes are divided into two groups can be considered as an example thereof. FIG. 9 shows an example where N segment electrodes are divided into M electrodes and (N-M) electrodes. The equivalent capacitance CB of a group (hereinafter referred to as the "B group") comprising M electrodes is c.multidot.M, and the equivalent output resistance rB thereof is ro/M. Further, the group (hereinafter referred to as the "A group") comprising (N-M) electrodes has an equivalent capacitance CA of c.multidot.(N-M) and an equivalent output resistance rA of ro/(N-M). The B group and the group A are switched to +Vs and -Vs and are connected by the switch SB and the switch SA, respectively. The display state for each row in each of these groups is assumed to be the same.
The results of a simulation using this example during the non-selection state are shown in the following drawings. In the drawings, symbols SWA and SWB denote the state of the switches SA and SB shown in FIG. 9. When SWA is at an H level, for example, the switch SA is connected to the +Vs side, and when it is at an L level, the switch SA is connected to the -Vs side. Symbols VDX, VSX, VMX, VA and VB represent the potentials or potential difference at the points shown in FIG. 9. In FIGS. 10 to 12, the relationship (N-M)&gt;&gt;M is established, to thus provide a condition whereby the influence due to the dullness of the waveform is noticeable, and values approximate to those of an actual display device are selected for ro, c, RD, RS and RM. Although the value c changes between the ON time and the OFF time in a practical liquid crystal, it is here assumed that the value c does not change in accordance with the state, for the purpose of simulation.
FIG. 10 shows the simulation results of the case that corresponds to FIG. 6. In FIG. 10, symbols VY1, VY2 and VY3 represent the selection timing of the common electrodes and this diagram shows the state where the selection potential (+Vc or -Vc) is applied to the respective common electrodes at the hatched portions while the zero (0) potential is applied thereto during the other periods. As these merely represent the timing, they are neglected during the simulation.
The state SWA of the switch SA described above changes to H and L whenever the selection period of the common electrodes changes, as shown in the diagram, because the A group described above must display only white or only black throughout the full row and the state SWB of the switch SB is fixed to H during one vertical scanning period, for example (to L during the next scanning period), because the B group should display each row alternately as white and black.
The waveforms of VA and VB in FIG. 9 at this time are ideally shown by VA and VB in FIG. 10, but in practice, these become VAX and VBX as shown in FIG. 10. Nevertheless, although the dullness of the waveform and spikes exhibit exponential changes in practice, they are expressed linearly for simplification. Furthermore, it is believed that the spike for an extremely short period can be neglected when calculating the effective value from the response of the liquid crystal, and thus this is omitted from the drawing (this also holds true for the subsequent drawings).
When VAX, VBX are compared with FIG. 6, it can be understood that VAX exhibits a similar tendency but VBX is apparently different. This is because VDX, VSX and VMX change as shown in FIG. 10, due to the presence of the resistors RD, RS and RM shown in FIG. 9. Next, this change will be explained. When the switch state SWA changes from L to H at the time Tp, a spike-like current flows from +Vs in FIG. 9 towards the zero (0) potential through the path ranging from the resistor RD, the switch SA, the resistor rA, the capacitance CA and the resistor RM, and the voltage drop due to this current changes VDX and VMX in the spike form. At this time, the current does not flow through the resistor RS, and VSX does not change. Next, when the switch state SWA changes from H to L at the time Tq, a spike-like current flows from the zero (0) potential towards -Vs through the path ranging from the resistor RM, the capacitor CA, the resistor rA and the resistor RS, so that VSX and VMX change. At this time, the current does not flow through the resistor RD, and VDX does not change.
If the value N-M is sufficiently high, the rA becomes sufficiently low. Therefore, the voltage drop due to the resistor rA is sufficiently smaller than the voltage drop due to the resistors RD, RS. On the other hand, the current flows through the capacitance CB with the change of VDX, VMX, but if M is sufficiently smaller than N-M, the value cB is sufficiently smaller than cA and the voltage drop component of the current flowing through cB due to the resistor rB becomes relatively very small. Namely, the voltage VAX (or VBX) across both ends of the liquid crystal is substantially VDX-VMX when the switch state SWA (or SWB) is at H and is substantially VSX-VMX when the switch state SWA (or SWB) is at L, as shown in FIG. 10.
In the vicinity of the time Tp, the changes of VDX and VDM act in a direction which reduces the effective values of both the VAX and VBX, but in the vicinity of the time Tq, the change of VAX acts in a direction that reduces the effective value for VBX and the changes of both VMX and VSX act in a direction that reduces the effective value for VAX. Accordingly, it is believed that the difference between VAX and VBX is affected more by the resistors RD, RS, RM than by the segment electrode output resistance ro in FIG. 8.
FIG. 11 shows the results of a simulation when the A and B groups described above effect white and black opposite displays throughout all the rows, and FIG. 12 shows the results of a simulation when the A group effects the white or black display but the B group effects a gray display between white and black. In these drawings, the symbols and names have the same meaning as in FIG. 10. The difference of these drawings from FIG. 10 is that the current resulting from the change of SWB flows through CB in FIG. 9, but this current component may be neglected because the value of CB is sufficiently smaller than the value of CA, as already described. Accordingly, the same concept as in FIG. 10 can be applied to FIGS. 11 and 12. Although an individual explanation thereof is omitted, it is obvious from these results that the driving effective voltage applied to the pixels of the A group drops at the time of a non-selection and the driving effective voltage applied to the pixels of the B group rises more than those of the A group at the time of a non-selection. Since the liquid crystal is assumed to be normally back, the display state becomes darker when the driving effective voltage drops and becomes brighter when the driving effective voltage increases. Therefore, the display state of a certain pixel in the A group becomes darker than its original display state, and the display state of a certain pixel in the B group becomes relatively brighter (brighter than the original display state in the cases of FIGS. 11 and 12, in particular). When the differences between the driving voltages VAX and VBX applied to the pixels of the A and B groups during the non-selection period are compared with one another for FIGS. 10, 11 and 12, it can be understood that the difference exists only near the time Tq in the case of FIG. 10, but the differences exist both near the time Tp and the time Tq in the cases of FIGS. 11 and 12. Naturally, the difference occurs in those rows in which the display state of the B group is different from the display state of the A group, and the difference of the driving effective voltage throughout the non-selection period is determined by the number of such rows.
The explanation given above deals with the non-selection period, and the situation becomes more complicated in the case of the selection period, as follows. If the dullness of the waveform of the selection voltage (.+-.Vc) is neglected, the A group gives a white display in FIG. 10, for example, the common electrode drive voltage VY1 should be -Vc at the time Tp, and therefore, -Vc-VDX is applied to the pixels of the Y1 row of the A group. Since the common electrode drive voltage VY2 should be +Vc at the time Tq, +Vc-VSX is applied to the pixels of the Y2 row of the A group. Obviously, the direction of the dullness of VDX, VSX in this case is the direction which reduces the effective value in the selection period (the direction which darkens the white). Conversely, when the A group effects the black display, the common electrode drive electrode VY1 at the time Tp should be +Vc. Therefore, +Vc-VDX is applied to the pixels of the Y1 row of the A group. Since the common electrode drive voltage VY2 should be -Vc at the times Tq, -Vc-VSX is applied to the pixels of the Y2 row of the A group. In this case, it is obvious that the direction of the dullness of VDX and VSX is the direction which increases the effective value in the selection period (the direction which brightens black). It can be assumed from the above discussion that the dullness of the drive voltage applied to the pixels of the A group during the selection period acts in such a direction as to lower the contrast. For the pixels of the B group, the same voltage as the voltage of the A group is applied to the pixels of the B group having the same display as the A group, but for the display pixels different from those of the A group, +Vc-VDX is applied at the time Tq when the A group effects the white display, for example, and the effective value during the selection period is not altered.
To summarize the above discussion, if N-M&gt;&gt;M in the example shown in FIG. 9, the following can be concluded.
(1) During the non-selection period, the driving effective voltage drops regardless of the display state in the A group. The degree of the voltage drop depends on the number of times of switching of the segment electrode voltage.
(2) During the non-selection period, the driving effective voltage increases more in the B group than in the A group regardless of the display state. The degree of this increase depends on the number of rows having a different display state from the A group at the time of switching of the segment electrode drive voltage of the A group.
(3) During the selection period, the driving effective voltage drops in the A group when the display state changes from black to white. The driving effective voltage increases when the display state changes from white to black.
(4) During the selection period, the driving effective voltage either increases, decreases or does not change in the B group, depending on the display state.
The driving effective voltage practically applied to the liquid crystal must be calculated throughout the selection period as well as throughout the non-selection period. Strictly speaking, therefore, an extremely complicated calculation must be made, depending on the display state. Therefore, it is assumed that the influences of the resistors RD, RS and RM are great as the cause of the cross-talk or the drop of the contrast. Accordingly, it must be concluded that the conventional concept is not sufficient, and thus really effective counter-measures cannot be taken.
FIGS. 11 and 12 show the results of a simulation wherein all the columns (N columns) of the liquid crystal display device are divided into two column groups (A group and B group) and the number N-M of the columns of the A group is made sufficiently greater than the number M of the columns of the B group, and FIG. 13 shows the results of a simulation where the number of the columns of the A group is the same as that of the B group. The timing relation in FIG. 13 corresponds to that of FIG. 11. Although the difference of the effective value between VSX and VBX is clearly observed in FIG. 11, the difference of the effective value between VAX and VBX is not observed in FIG. 13. Namely, although cross-talk does not occur in this case, it is important to note that the dullness of the waveform at each part in FIG. 13 is smaller than that in FIG. 11. As already described, the effective value at the time of selection is affected by the dullness of the waveforms of VDX and VSX. Since the dullness of VDX and VSX is great in the case of FIG. 11, the deviation of the driving voltage applied to the pixels during the selection period from the theoretical value is great, and the tendency for the white portion to become dark and the black portion to become bright is strong, so that the contrast drops even when the effective value during the non-selection period is the same. In the case of FIG. 13, however, the dullness of VDX, VSX is small and the drop of the contrast is also small. Paradoxically, the maximum contrast can be obtained by displaying half of the screen in white and the other half in black; if the screen is displayed as fully white or black and the difference between these cases is considered, the lowest contrast can be obtained.
The cause of the difference of the dullness of the waveform between FIGS. 11 and 13 can be understood to be as follows. FIG. 14(A) is an equivalent circuit diagram when the case of FIG. 13 is used as an example is assumed that the capacitance of the liquid crystal formed by the half of the screen is cx and RD, RS and RM are all rx, for a simplification. At this time, the current Ix flowering from +Vs flows to -Vs and the current does not flow towards the zero (0) potential. The time constant Tx of the circuit at this time is (2.multidot.rx) (cx/2)=rx cx, and the dullness of the waveform of the voltage applied across both ends of the liquid crystal of the A group is small, as shown in FIG. 14(B).
Further, FIG. 15(A) is an equivalent circuit diagram corresponding to FIG. 11. Assuming that the capacitance of the B group is much smaller than that of the A group, the capacitance of the A group can be set to 2.multidot.(cx) by regarding the capacitance of the B group as zero (0). At this time, the current flowing from +Vx flows fully towards the zero (0) potential. The time constant Tx of the circuit at this time is (2.multidot.rx).multidot.(2.multidot.cx)=4.multidot.rx.multidot.cx, and the dullness of the waveform of the voltage applied across both ends of the liquid crystal of the A group becomes four times as great as that of FIG. 14, as shown in FIG. 15(B). This difference of the time constants means that the difference of four times also exists between the maximum and minimum dullness of the waveforms of VDX and VSX.
Assuming that the effective value at the time of non-selection is equal, the difference of the display state is determined by the difference of the effective values at the time of selection, and since the effective value at the time of selection is affected by the dullness of the waveforms of VDX, VSX, the difference of the dullness of the waveforms VDX, VSX at the time of selection means the difference of the effective value at the time of selection. Accordingly, the portion which should originally have the same brightness becomes different depending on the display state. When the effective values are calculated, the difference of four times of the time constants is a value greater than four times.
A counter-measure for the cross-talk which takes the resistance of the power supply lines into consideration has been very recently proposed ("SID 90 DIGEST, 412.21: "Crosstalk-Free Drive Method for STN-LCDs" (hereinafter referred to as the "Reference 2")). FIG. 3 of this reference depicts the resistor corresponding to RM of the present invention, and the Reference 1 ascribes the voltage drop due to this resistor as one of the causes of the cross-talk. To correct the influences of this voltage drop, the Reference 1 adds a D.C. bias voltage .DELTA.V to VM, which is defined in the present invention, in each drive period of each row. The Reference 1 describes that the .DELTA.V at this time can be calculated from the difference between the number of pixels in the ON state on the common electrodes which are now in the selection period, and the number of pixels which are to be turned ON, on the common electrodes which are to be selected next.
To state the conclusion first, this method is effective as a counter-measure for the cross-talk, but this example does not fully consider the power supply resistors RD, RS in the present invention. Since the Reference 2 is based on the concept that the difference of the effective voltage during the non-selection period is offset by the D.C. bias, the value .DELTA.V described above is relatively small and the dullness of the waveform does not change much. This means that, although the effective voltage during the non-selection period can be made uniform, the influence of the dullness of the waveform during the selection period is not greatly improved and the cause of the cross-talk remains. In connection with the contrast and response also, improvements are yet to be made as long as the influences of RD and RS exist. As described in the Reference 2, this method is effective for a "frame gradation", but cannot be applied so easily to those devices which effect a gradation display by changing the voltage impression time during the selection period, as in a liquid crystal television receiver.
The value .DELTA.V described above is calculated from the difference of the number of the pixels in the ON state on the common electrodes that are currently in the selection period, and the number of the pixels, which are to be turned ON, on the common electrodes which are to be selected next. Nevertheless, even though the number of the pixels turned ON during a certain selection period is the same, the timing at which the pixels are turned ON is not always the same. In other words, there is the case where all the pixels are simultaneously turned ON, and there is also another case where the pixels are turned ON individually or non-uniformly. Since the effective values turn out different in both of these cases, a correction cannot be made. Since the capacitance of the liquid crystal changes with the drive state, as already described, the capacitance value, or the current value and thus the power supply voltage change, changes in a complicated manner under a complicated drive state such as in the case of the gradation display, and it is extremely difficult to maintain a predetermined correction state. The change of the ambient temperature also must be taken into consideration when solving this problem.